Smart contracts, Nethereum and truffle

In my previous post, Ethereum blockchain interaction with Nethereum I’ve hardcoded the contract code and ABI inside the dotnet client. It looked bad, and I didn’t like it!

Also, I did not use truffle testing capabilities as I had issues with ganache-cli not propagating events.

To start, I’ve created an empty folder, and ran:

truffle init

After that, I added my contract in the contracts folder. I did not like the original structure so I removed the constructor parameter and changed the event to have the multiplicand, multiplier and product parameter names.

pragma solidity ^0.4.21;

contract Multiplier {
    event Multiplied(uint256 indexed multiplicand, uint256 indexed multiplier, address indexed sender, uint256 product);

    function multiply(uint256 a, uint256 b) public returns(uint r) {
        // looks like an overflow waiting to happen
        r = a * b;
        emit Multiplied(a, b, msg.sender, r);
        return r;
    }
}

adding tests

Next, I added simple sanity checks to my contract. The contract artifact first needs to be require’d:

var Multiplier = artifacts.require("./Multiplier.sol");

Then you can create an instance of this contract by calling the deployed() method :

contract = await Multiplier.deployed();

After that you can write tests! Truffle js testing comes with async/await support, which means shorter and more conprenhensible code!

it("should multiply", async function () {
    let res = await contract.multiply.call(4, 5);
    assert.equal(res, 20);
});

At first, the tests would fail with a cryptic error.

  1) Contract: Multiplier with deployment "before each" hook: deploy contract for "deploy":
     Error: Multiplier has not been deployed to detected network (network/artifact mismatch)
      at /usr/local/lib/node_modules/truffle/build/webpack:/~/truffle-contract/contract.js:454:1
      at <anonymous>
      at process._tickCallback (internal/process/next_tick.js:188:7)

It turns out I forgot to add a 2_deploy_contract.js migration file in the migrations folder :

var Multiplier = artifacts.require("./Multiplier.sol");

module.exports = function(deployer) {
  deployer.deploy(Multiplier);
};

Below is the full test code. Of course, a lot more tests could be expected in production code. This one being a sample I’ve kept it concise.

var Multiplier = artifacts.require("./Multiplier.sol");

ccontract('Multiplier', function () {
  let contract = null;

  beforeEach('deploy', async function() {
    contract = await Multiplier.deployed();
  })

  describe("multiply", function() {
    it("should multiply", async function () {
      var res = await contract.multiply.call(2, 2));
      assert.equal(res, 4);
    });
  });
});

compiling contract

Running the truffle compile command will produce the bytecode and ABI, in a json file. By default, this file is located in the build/contracts folder of your contract project. This can be changed in the truffle.js file.

module.exports = {
  // See <http://truffleframework.com/docs/advanced/configuration>
  // to customize your Truffle configuration!
  contracts_build_directory: "./../../Client/NethClient/Contracts/Multiplier"
};

loading the contract

To copy the contracts to output folder, alter the csproj file by adding this :

  <ItemGroup>
    <None Include="Contracts/**" CopyToOutputDirectory="Always" />
  </ItemGroup>

Now that the contract is present in the dotnet project as well it can be loaded using a JObject.

var json = JObject.Load(new JsonTextReader(new StreamReader(File.OpenRead("Contracts/Multiplier/Multiplier.json"))));
var abi = json["abi"].ToString();
var bytecode = json["bytecode"].ToString();

I took the opportunity to refactor the code a little bit to further separate contract manipulation in another class. This could be useful for later use with other contracts.

be careful with those events and gas

After running the code, everything went fine, except no events were being returned.

client_1  | Multiply 8 by 9 using 24082 gas
geth_1    | INFO [05-15|20:01:08] Submitted transaction                    fullhash=0xdfde494ea6d244f927066bf877862b400e0b358bfba33bb790a907736a4d1a1c recipient=0x98245fba911B71ECAe3490ba9D324a2C7038ffbB
geth_1    | INFO [05-15|20:01:13] Successfully sealed new block            number=361 hash=377574…fffd4f
geth_1    | INFO [05-15|20:01:13] 🔗 block reached canonical chain          number=356 hash=281662…94a4fd
geth_1    | INFO [05-15|20:01:13] 🔨 mined potential block                  number=361 hash=377574…fffd4f
geth_1    | INFO [05-15|20:01:13] Commit new mining work                   number=362 txs=1 uncles=0 elapsed=15.164ms
geth_1    | INFO [05-15|20:01:18] Successfully sealed new block            number=362 hash=460dc2…306b71
geth_1    | INFO [05-15|20:01:18] 🔗 block reached canonical chain          number=357 hash=cd3d35…0435c3
geth_1    | INFO [05-15|20:01:18] 🔨 mined potential block                  number=362 hash=460dc2…306b71
geth_1    | INFO [05-15|20:01:18] Commit new mining work                   number=363 txs=0 uncles=0 elapsed=1.468ms
client_1  | -> Done at block 362 using 24082 gas
client_1  | Get all events for filter 317348022035050220953892121001492572177
client_1  | -> Got 0

It turned out I forgot one parameter in the gas calculation function.

// wrong
var gas = await _multiply.EstimateGasAsync(multiplicand).ConfigureAwait(false);

// correct
var gas = await _multiply.EstimateGasAsync(multiplicand, multiplier).ConfigureAwait(false);

Gas was lower than required, and no event was fired. Which is strange, because if gas was not sufficient, an error should have been raised ? After fixing the code :

client_1  | Multiply 8 by 9 using 24274 gas
geth_1    | INFO [05-15|20:15:28] Submitted transaction                    fullhash=0x1454c7801c61fe19f9950976418a6af1b0e696317f77cfbf5ab65cbbd124c653 recipient=0x68281a51523A439260Ca30c7121D6Ed9B39A95e6
geth_1    | INFO [05-15|20:15:33] Successfully sealed new block            number=379 hash=dd9754…a4e240
geth_1    | INFO [05-15|20:15:33] 🔗 block reached canonical chain          number=374 hash=5d3b4b…f477a3
geth_1    | INFO [05-15|20:15:33] 🔨 mined potential block                  number=379 hash=dd9754…a4e240
geth_1    | INFO [05-15|20:15:33] Commit new mining work                   number=380 txs=1 uncles=0 elapsed=4.295ms
geth_1    | INFO [05-15|20:15:38] Successfully sealed new block            number=380 hash=64b91a…0bc39b
geth_1    | INFO [05-15|20:15:38] 🔗 block reached canonical chain          number=375 hash=300263…5cc25a
geth_1    | INFO [05-15|20:15:38] 🔨 mined potential block                  number=380 hash=64b91a…0bc39b
geth_1    | INFO [05-15|20:15:38] Commit new mining work                   number=381 txs=0 uncles=0 elapsed=3.007ms
client_1  | -> Done at block 380 using 24274 gas
client_1  | Get all events for filter 189326955700024171316112267564238036559
client_1  | -> Got 2
client_1  | -> Block 378 : 7 * 7 = 49
client_1  | -> Block 380 : 8 * 9 = 72

adding more tests

I’m going to add chai, chai-as-promised and chai-bignumber.

require('chai')
  .use(require('chai-as-promised'))
  .use(require('chai-bignumber')(web3.BigNumber))
  .should();

var Multiplier = artifacts.require("./Multiplier.sol");

contract('Multiplier', function () {
  let contract = null;

  beforeEach('deploy', async function() {
    contract = await Multiplier.deployed();
  })

  describe("multiply", function() {
    it("should multiply", async function () {
      (await contract.multiply.call(2, 2)).should.bignumber.equal(4);
    });

    it("should not overflow", async function () {
      // uint256 max value
      const max = new web3.BigNumber(2, 10).pow(256).sub(1);

      await contract.multiply.call(max, 2).should.eventually.be.rejected;
    });
  });
});

Running the tests should fail because of the overflow error :

$ truffle test
Compiling ./contracts/Migrations.sol...
Compiling ./contracts/Multiplier.sol...
Compiling openzeppelin-solidity/contracts/math/SafeMath.sol...

  Contract: Multiplier
    multiply
      ✓ should multiply simple numbers (66ms)
      ✓ should multiply BigNumbers (57ms)
      1) should not overflow
    > No events were emitted


  2 passing (286ms)
  1 failing

  1) Contract: Multiplier multiply should not overflow:
     AssertionError: expected promise to be rejected but it was fulfilled with { Object (s, e, ...) }

referencing openzeppelin

Next, I added openzeppelin with:

yarn add openzeppelin-solidity --save-dev

Now, I can reference this library by importing the desired contract in my contract code file. Here I used the SafeMath library :

import "openzeppelin-solidity/contracts/math/SafeMath.sol";

contract Multiplier {

    event Multiplied(uint256 indexed multiplicand, uint256 indexed multiplier, address indexed sender, uint256 product);

    function multiply(uint256 a, uint256 b) public returns(uint r) {
        r = SafeMath.mul(a, b);

        emit Multiplied(a, b, msg.sender, r);

        return r;
    }
}

And now, running the tests again should pass :

$ truffle test
Compiling ./contracts/Migrations.sol...
Compiling ./contracts/Multiplier.sol...
Compiling openzeppelin-solidity/contracts/math/SafeMath.sol...

  Contract: Multiplier
    multiply
      ✓ should multiply simple numbers (52ms)
      ✓ should multiply BigNumbers (41ms)
      ✓ should not overflow

  3 passing (234ms)

Everything looks fine now.

The source repository for this article is on Github.